Immunoneutralization of circulating inhibin in the hypophysiotropically clamped male rhesus monkey (Macaca mulatta) results in a selective hypersecretion of follicle-stimulating hormone.

The purpose of this experiment was to examine directly whether inhibin is involved in the testicular regulation of FSH secretion in the male rhesus monkey. To this end, the pituitary-testicular axis in eight juvenile monkeys was prematurely activated by a chronic iv infusion of GnRH (0.1 microgram/min for 3 min every 3 h). After a minimum of 5 weeks of pulsatile GnRH stimulation, four animals received a brief (30-min) iv infusion of an ovine antiserum to inhibin alpha-subunit (approximately 10 ml/kg BW), and four monkeys received a comparable volume of a control ovine immune serum. The pulsatile GnRH infusion continued without interruption throughout the entire experiment. The FSH response to passive immunization against inhibin was determined by measuring concentrations of this gonadotropin in sequential plasma samples collected immediately before a GnRH infusion and for 3 h thereafter (an inter-GnRH pulse interval) on days 0.5, 1, 2, 4, 8, and 16 after injection of the immune serum. Administration of the antiserum to inhibin alpha-subunit resulted, within 2 days, in a 2- to 3-fold increase in the mean concentration and pulse amplitude of plasma FSH. The hypersecretion of FSH induced by administration of the antiserum to inhibin alpha-subunit was maintained until day 4, and then mean concentrations and mean pulse amplitudes of plasma FSH declined, reaching preantibody control levels by day 16. The time course of the antiserum-induced hypersecretion of FSH was closely correlated to changes in circulating inhibin-binding activity. Most importantly, the hypersecretion of FSH observed during the first 2 days after immunoneutralization of circulating inhibin was indistinguishable from that elicited during the initial 2 days after subsequent bilateral orchidectomy and concomitant testosterone (T) replacement. Administration of a control immune serum did not influence circulating FSH concentrations, and neither the antiserum to inhibin alpha-subunit nor the control immune serum induced changes in pituitary LH secretion and testicular T release. Since the exogenous drive to the pituitary-testicular axis of the animals was clamped in a mode that produced a pattern of pulsatile LH and T secretion comparable to that observed in adult monkeys, the present findings provide evidence for the view that inhibin plays a major role in the testicular regulation of FSH secretion during adulthood by exerting a selective inhibition on the secretion of this gonadotropin directly at the level of the anterior pituitary gland.